LONGSHORE SAND TRANSPORT

Abstract

Various reliable data sets from sites (USA and Netherlands) have been selected to analyse the longshore transport process and to establish the relationship between wave height, wave incidence angle and longshore transport, yielding a relatively simple transport formula. The data set was too small to detect any influence of particle size, wave period and profile shape. These aspects were studied by using the results of a detailed process-based model, which were parameterized and implemented in the simplified formula. The main overall result is a general expression for longshore transport of sand and gravel, including the effects of profile slope and tidal velocity. INTRODUCTION The computation of a reliable estimate of longshore sand transport remains of considerable practical importance in coastal engineering applications such as the derivation of sediment budgets for coastal areas with and without structures (breakwaters, groynes) and long-term beach stability with and without beach nourishments or coarse-grained beach protections. Most research on longshore transport has concentrated on sand sized sediment, but research on longshore transport along gravel and shingle beaches has also been performed to deal with the erosion problems along these types of beaches, which are quite common along midand high-latitude (formerly glaciated) parts of the world. The most widely used formula for longshore transport (LST) is commonly known as the CERC equation (Shore Protection Manual, US Army Corps of Engineers, 1984). This method is based on the principle that the longshore transport rate (LST, incl. bed load and suspended load) is proportional to longshore wave power P per unit length of beach; LST=K P, with K=calibration coeffcient. The CERC formula has been calibrated using field data from sand beaches. The effects of particle diameter and bed slope have been studied systematically by Kamphuis (1991), resulting in a more refined equation for longshore sediment transport. This latter equation was found to give the best agreement between computed and measured transport rates (Schoonees and Theron, 1996). Both equations (CERC and KAMPHUIS) have been used in the present study. Furthermore, a detailed process-based model (CROSMOR2000) has been used in this study to compute the longshore sand transport distribution along the cross-shore bed profile. First, this model 1 Senior engineer, Delft Hydraulics, PO Box 177, 2600 MH Delft, Netherlands; leo.vanrijn@wldelft.nl.